Method and apparatus for measuring force factors



Feb N, l9"- l.. A. MORRISON TAL 2,231,085

METHOD AND APPARATUS FOR MEASURING .FORCE FACTORS Filed June 3, 1959 2 Sheets-Sheet 1 l )NVENTORSIAAjg//SON ATTORNEY Feb. 1l, 1941. L A MORRSON ErAL 2,231,085

METHOD AND APPARATUS FOR MEASURING FORCE FACTORS Filed June 5, 1959 2 Sheets-Sheet 2 .1. .A Mom/50N /NVENTOEEMOTT A TTORNE Y Patented Feb. ll, 1941 y UNITED .STATES PATENT oEFlcE i ME'rnon Arm ArrAaA'rUs ron nEAsUanvc ronca FAc'roas Louis A. Morrison, Madison, and Edward E. Mott.

Upper Montclair, N. J., assignors'to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation o! New York Application June s, 1939, sexismo. 211,128 v llclaims.

This invention relates to a method of and apparatus for measuring the force factors of electromechanical systems.

This application is a continuation in part oi applicants copending applications, Serial Nos.`

for electroacoustic transducers may be expressed KA Q). Y y M (for constant amplitude devices) (1) CA( G 5 1/ Z JM (for constant velocity devices) (1a where: =gure of merit K and C:constants A=effecve area of acoustic radiator G=force factor M:eiective mass of moving system Z:damped electrical impedance of transducer.

From the above expression it will be noted that for systems for the same general purpose, for example, cone type loud-speakers or telephone 3 receivers, the expressions The foregoing object is attained by the method and apparatus of this invention which comprises producing a relative displacement of a knowny amount between the movable and stationary members of the electromechanical system. and

measuring the ow of electric charge produced thereby. e'

The forcefactormay be expressed mathematically as: i

G'== =gg=approximately where: G=force factor (dynes/abampere) F=instantaneous force in dynes i :instantaneous current in abamperes y e :instantaneous voltage 10 v :instantaneous velocity in centimeters/second Ame) :incremental change in total flux linkages A:c:incremental displacement of movable system.

In` the foregoing expression if Amo) and Arr are made very small their ratio approaches 15 as is weil known. In this invention a carefully controlled incremental displacement is made use 20 of to yield .an accurately measured value of the force factor as will appear from the more detailed description which is to follow taken in connection with the accompanying drawings in which: 25

Fig. 1 discloses the fundamental features of one application of the invention to the measurement of the force factor of an electromagnet and armature;

Fig. 2 shows the invention applied to a moving 3U coil type System;

Fig. 3 is a specific practical embodiment of the invention applied to telephone receiver diaphragms wherein the incremental displacement is eiected by shifting `the magnet with respect 35 to the diaphragm;

Fig. 4 is a top View of a partial section of Fig. 3;

Fig. 5 is a bottom view of the control knob as seen from section 5 5 of Fig. 3; and

Figs. 6, 'I and 8 are three views of a diierent 40 speclilc embodiment also for testing telephone receiver diaphragme wherein the incremental dis placement is effected by shifting the receiver diaphragm.

Returning to Fig. 1, an electromagnet I which d5 `example to dotted position l', there will be a corresponding change in flux linkages due to the a5 change in reluctance of the magnetic circuit. It will be understobd that the increment Aar as shown is greatly exaggerated and is actually a relatively small distance, for example to 1 mil.

A coil 3 is wound on a portion of magnet l and receives current from battery 4 to generate the necessary magnetomotive force. An ammeter 5 and rheostat 6 provide easy control. A coil I6 is mounted on each of the poles 2 and the current induced therein is a measure of the change of flux linkages Ami. Coils I6 are serially connected to ballistic galvanometer 8 through a suitable shunt 9 and switch III when the latter is connected to contact II.

Switch I0 may also be connected to contact whereby a portable search coil I5 is connected to galvanometer 8 through the flexible two-conductor lead I3. A suitable magnetic core I4 is provided for coil I5. This coil is used to adjust the initial distance S as will be explained more in detail later.

In Fig. 2 the electromagnet I is shown schematically in the form of an E instead of the U of Fig. 1. This form is selected here because it provides a working gap essentially like that of the field of a moving coil dynamic loud-speaker and is therefore especially convenient for measuring the force factors of moving coil systems. 'I'he center leg I8 of the E is shown as havingyan enlarged section I1 of suitable length to limit the initial position of the movable coil I9. This, of course, is not the only way the initial position can be limited and in actual practice leg I8 may be of uniform diameter and carry an adjustable stop for limiting the initial position of the coil in a manner analogous to those hereinafter more particularly disclosed in Figs. 3 and 8 for telephone receiver diaphragms. Coil I9 is usually wound on a hollow support I9 and in the case of most loud-speakers this is a light fiber form to be attached to the core or diaphragm. In this embodiment coil I9 acts in a manner analogous to coils I6 of Fig. 1. Coil 3, battery 4, ammeter 5, rheostat 6, galvanometer 8 and shunt 9 are as in Fig. 1. The coil mounting means schematically represented herein as wires 28 and 2l are adapted to transmit to coil I9 an incremental displacement Az.

Figs. 3 and 4 disclose a particular practical embodiment of the mechanical features of Fig. 1 adapted for a telephone receiver diaphragm. In this embodiment the incremental displacements are effected by moving the magnet poles 2 with respect to the diaphragm which is kept stationary. In order to reduce the mass to be moved. the electromagnet I is divided into parts I and I. The U-shaped part I is an electromagnet with energizing coil 3 while the two bar-shaped parts I' constitute two parallel mounted permalnent magnets of high retentivity terminating in soft iron poles 2. 'Ihis latter structure is essentially that used in telephone receivers more particularly disclosed in Fig. 6 of either of applicants copending applications, Serial No. 93,792 or 161,936. f

Electromagnet I is slidably supported in a pair of slides 23, 23 under a horizontal supporting plate 22 of non-magnetic material, preferably brass. Slides 23, 23 may be attached tov plate 22 by means of screws 24. When the electromagnet I is slid into the position indicated in Fig. 4 its poles are closely adjacent the ends of permanent magnets I'. 'I'he magnetism retained by magnets I' is then under the control of electromagnet I by adjusting rheostat 6 shown in Fig.

machined so it is in the same plane as the top faces of poles 2 when the latter are raised.

APlate 22 is fixed to the top of a second plate 21 by means of three short posts 28. Plate 21 is in turn rigidly supported by three long posts 29 fixed to a base 30.

Magnets I with their poles 2 and coils I6 are mounted on a flange 3i which is movable in a vertical direction by micrometer 32 through rod 33. VA slidable joint 34 permits rotation of micrometer 32 while rod 33 is prevented from rotation byl a keyway 35 and screw 36. A spring 31 urges flange 3| upwardly to remove any backlash orlooseness in joint 34 or the threads in micrometer 32. Rod 33 is given a close sliding t in a hole bored vertically through a column 38 which is rigidly mounted on `the under-side of plate 21. l

Micrometer 32 is of conventional design with the stationary part 39 held rigidly in the hub of a supporting disc 4I by means of set screw 40. Disc 4I is rigidly attached to the underside of column 38 and around its periphery a rotatable measuring ring42 is mounted so as Ato have a close sliding fit. Ring 42 is retained by a collar 43 as indicated and a knurled thumb screw 44 is provided to adjustably x its position. A downwardly disposed extension 46 integral with ring 42 carries two parallel rods 41, better shown in Fig. 5, which together with arm 49 act as stops to restrict the angular rotation of micrometer 32 to a small angle. Arm 49 is secured to micrometer 32 through a knob 48 and set screw 50. The effect of this small angular rotation is to provide a fixed small incremental displacement of known magnitude for the magnets I and their poles 2. In making studies over wide ranges of initial air-gaps it is helpful to use an auxiliary scale which is more easily read. 'I'his auxiliary scale 50 is engraved on a separate annulus 5I frictionally mounted within disc 4I and movable by a pin 52. A reference index 53 is engraved on a beveled portion 45 of ring 42. Each scale division of scale 5I may represent 1 mil or other suitable small unit of measure. In using the auxiliary scale thumb screw 44 is loosened and measuring ring 42 is rotated the desired number of revolutions or fractions thereof as measured by scale 5I, after,

clarity. A portable search coil I5 and switch III may also be provided.

To prepare Athis apparatus for measuring the force factor of a telephone receiver diaphragm operating under various magnetic intensities and varying air-gaps, it is first necessary to accurately establish an initial air-gap S as shown in Fig. 1. This` is most conveniently accompushed by an indirect magnetic measurement employing a portable search coil,such as coil I5 of Fig. 1. "Ihe poles of core' Il are made with the same pitch as poles 2 so that when core Il is placed on poles 2 they form a completed magnetic circuit with negligible air-gap. Poles 2 are raised ilush with or slightly protruding through diaphragm support 25 and a non-magnetic material such as phenol ber of accurately measured thickness is inserted between core I4 and poles 2. Magnets. I' are given any convenient magnetization by means of electromagnet I after which electromagnet I is withdrawn from their ilelds. The'thickness of the phenol liber spacer between poles 2 and core Il is preferably substantially equal to the normal working gap for the diaphragm to be tested. Switch Il is connected to contact I2. Search coil I5 and its core Il are quickly withdrawn and the resulting ballistic deiiection of galvanometer 5 is noted.

Screw M is then' released and poles 2 are lowered by rotating ring I2. With the spacers removed the above-described ballistic reading is repeated until the deilection is the same as be.

fore whereupon the gap S is equal to the nonmagnetic spacer thickness. lScrew I4 is thereupon tightened. Although the initial gap S has been adjusted by the above-described indirect magnetic means, it is obvious that other wellknown direct means may be used such es depth gauges commonly used by machinists.

After the above adjustment of initial gap has vineen made it is apparent that any other mitm Vgap distance may be obtained by loosening nut 44 and rotating ring I2 the desired amount according to the auxiliary scale 5I.

For any initial gap setting the change of flux linkagesl MM) may be measured by turning switch I6 to contact II, swinging micrometer 32 once' through the restricted arc determined by posts 41 and arm I9 and noting the ballistic response of galvanometer 8. From the readings thus obtained and the calibration -of the galvanometer and coil circuit employing a mutual inductance standard in a well-known manner..

the value of Mno) is obtained. Mathematically the value of A(n 1 is:

factor may be measured for varying magnetic intensities by changing the retained magnetism of magnet I'.

Figs. 6, 'I and 8 disclose a device which, although quite diiferent mechanically, is identical in principle to the one disclosed in Figs. 3, 4 and 5. In this embodiment the diaphragm is displaced instead of the magnet and pole-pieces as in Fig. 3 and the Vdisplacement is effected by means of wedges 10 and 14 instead oi' by a micrometer.

'Ihe diaphragm support 25 is here supported by the cylinder 56 having a cavity 51 in its top vbored out to take a cylindrical reduced shaft 6I extending upwardly from cylinder 66. Shaft 6I 10 is accurately machined at raised points 62 and 63 to provide a close sliding fit with the bore in cylinder 56. A screw 58 is threaded into the top` of shaft 6I and with spring 59 provides positive sliding contact between cylinder 56 and wedge 14.

. A horizontal slotV is cut through cylinder 60 to take wedge 14 as shown in Fig. 8. A cut-away bottom portion of cylinder 56 rests on wedge 14 so that as wedge 14 is slid back by pushing on handle 15, cylinder 56 is raised a small increment. v'I'his provides the necessary incremental displacement Ax. Set screws 16 and 11 provide accurate adjustment of the magnitude of 25 Az.

'Ihe bottom of cylinder 60 is also boredV out to take afcylindrical reduced shaft 65 and has accurately tted surfaces 66 and 61 similar to surfaces 62 and 63. Spring 69 is similar to spring 59 and screw 68 is threaded into cylinder 60. 3 Shaft 65 is integral with cylinder 6B which, in turn, is secured to the legs of electromagnet I as shown in Fig. 6.

Wedge 10 is inserted through a slot in cylinder 6I and provides easy adjustment for the initial 35 gap S which may be varied by adjusting opposing screws 1I and 12. The wedge-shaped recesses 13, 13' are cut in cylinder 6d to insure that turning screws 1I and 12 will not raise wedge 10. 40

It is evident from the foregoing descriptions that an electric charge is caused to ow in the circuit of coils I6 whenever there is a relative displacement between either the diaphragm 1 and poles 2 in Fig. 1 or the coil I9 and the poles of electromagnet I in Fig. 2 and that this ow is measured by the changeof uxlinkages Ama) incoils I6.

To measure the force factors of other electro? magnetic systems. for example relays or magnetic switches, it is only necessary to adapt the dimensions and shapes of the various parts of either Fig. 3 or Fig. 8 to .the particular device which it is desired to test. The manner of doing this ls obvious to any one skilled in the art. In the particular case of magnetic switches, the electromagnet I of Fig. 1 may be iield magnet of'4 the switch and the armature 1 will, of course, be the switch armature.

It is to be noted that anhough for each of the 6 specic embodimentsl herein disclosed the displacement Aa: is adjustable in magnitudayet for practical measurements .this displacement is kept constant. Hence, according to Equations 65 2 and 3 it is evident that the force factor G is measured directly by the deflection Dr in the practical apparatus.

What is claimed is:

1. A method of measuring the force factor of 70 an electromechanical system having a normally movable and a normally stationary member comprising moving the normally movable member of said system through a unidirectional single transit of known amount with respect to the nor- 75 measuring the ilow in electric chargegproduced by said movement.

' 2. A method of measuring .the force factor oi' an electromechanical system having a, normally movable and a normally stationary member comprising displacing the normally stationary member through a unidirectional single transit of known amount with respect to the normally movable member of said system, and measuring .the iow in electric charge produced by said displacement.

3. A method of measuring the force ,factor ofan electromechanical Vsystem having a normally movable and a normally stationary member comprising effecting a single unidirectional relative displacement of known magnitude between said two members, and measuring the resultant flow of electric charge.

4. A device for measuring the force factor of an electromechanical systeml having a normally movable and a normally stationary member comprising'means for mechanically displacing one of said members through a unidirectional single transit of known amount relative to the other, electrical means associated with said system adapted to transform the change in mechanical energy resulting from said displacement into an equivalent ow of electric charge, and measuring means for measuring said ow of electric charge.

5. A device for measuring the force factor of an electromechanical system having a normallymovable and a normally stationary member comprising means for mechanically displacing saidnormally movable member through a unidirectional single transit of known amount with respect to said normally stationary member, electrical means associated with said system adapted to transform the change in mechanical energy resulting from said displacement into an equivalent flow of electric charge, and measuring means for measuring said ow of electric charge.

6. A device for measuring the force factor of an electromechanical system having a normally movable and a normally stationary member comprising means for mechanically displacing said normally stationary member through a. unidirectional single transit, of known amount with respect to said normally movable member, electrical means associated with said system adapted to transform the change in mechanical .energy resulting from said displacement into an equivalent flow of electric charge, and measuring means for measuring said flow of electric charge.

7. A device for measuring the force factor of a telephone receiver diaphragm movable in a given magnetic circuit, means for producing a constant magnetomotive force in said magnetic circuit, displacing means operatively engaging said diaphragm and adapted to move it a known distance thereby changing the reluctance of said magnetic circuit, a coil associated with said magnetic circuit responsive to the change of ilux linkages produced bysaid reluctance change, and a measuring means cooperating with said coil to quantitatively indicate the said change of flux linkages. 8. A device for measuring the force factor of a .telephone receiver diaphragm comprising a magnetic circuit including said diaphragm as a part thereof, means for producing a. magnetomotive force in said magnetic circuit, means for mechanically displacing said diaphragm a small distance with respect to the rest of the magnetic circuit whereby the reluctance of the magnetic circuit is changed,` electrical means associated with said magnetic circuit responsive to the change in flux vresulting from said reluctance change, and a measuring means cooperating with said electrical means to quantitatively indicate the said change in-ux.

9. A device for measuring the force factor of an electroacoustic transducer comprising a magnetic circuit, means for producing a magnetomotive force in said circuit, a movable driving element for said transducer, means for accurately positioning said driving element within the eld of said magnetic circuit, mechanical displacing means adapted to eiect a small known relative displacement between said driving element and said magnetic circuit whereby a flow of electric charge is produced, and means for measuring the ow oi' electric charge.

10. A device for measuring the force factor of an electroacoustic transducer, a coil for driving the acoustic radiator of said transducer, a magnetic circuit, means for producing a magnetomotive force in said circuit, means for accurately positioning said coil within the iield of said circuit, mechanical displacing means adapted to effect an incremental relative displacement between said driving coil and said magnetic circuit thereby changing the flux linkages in said coil, and means for quantitatively indicating the said change in flux linkages.

11. A device for measuring the force factor of an electromechanical system having a normally movable and a normally stationary member comprising a magnetic circuit, means for producing a magnetomotive force in said circuit, means for adjusting the magnitude of said magnetomotive force, means for accurately positioning said movable member with respect to said stationary member and within .the field of said circuit, an electric coil linking said magnetic circuit, a mechanical displacing means adapted to eiect an incremental relative displacement between said two members thereby changing vthe ilux linking said coil, and means for quantitatively indicating said change in ux linkages.

LOUIS A. MORRISON. EDWARD E. MO'IT. 

